The invention relates generally to volumetric imaging systems and more particularly to methods and device for the geometrical analysis and calibration of volumetric imaging systems.
Volumetric imaging systems are utilized for various applications in both medical and non-medical fields. These imaging systems may include for example C-arm, tomosynthesis, computed tomography (CT) imaging systems having varying topologies and are used to create volumetric images or views of a patient based on the attenuation of radiation passing through the patient. Based on the attenuation of the radiation, the topology of the imaging system, and the type and amount of data collected, different views may be constructed, including views showing motion, contrast enhancement, volume reconstructions, two-dimensional images and so forth. Alternatively, volumetric imaging systems may also be utilized in non-medical applications, such as in industrial quality control or in security screening of passenger luggage, packages, and/or cargo. In such applications, acquired data and/or generated images representing volumes or parts of volumes (e.g., slices) may be used to detect objects, shapes or irregularities which are otherwise hidden from visual inspection and which are of interest to the screener.
Some volumetric imaging systems, both medical and non-medical, utilize a radiation source to generate the radiation beam such as X-ray beam and a detector to detect the attenuated signals. The source and/or the detector may move with respect to each other and/or the imaged object, or they may remain stationary. The beam passes through the object being imaged, such as a patient. The radiation beam, after being attenuated by the object impinges upon a detector consisting of an array of detector elements. The intensity of the radiation received at the detector array is dependent upon the attenuation of the radiation beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam intensity at the detector element location. The intensity measurements from all the detector elements are acquired separately to produce a transmission profile, commonly referred to as projection image.
With the introduction of multi-row and volumetric imaging scanners, including gantry-based and benchtop-type scanners, it has become necessary to determine critical alignment parameters beyond those necessary for traditional two-dimensional scanners. Without these critical alignment parameters, it is difficult or impossible to obtain adequate image quality from a scanner, as the image reconstruction process requires an accurate knowledge of scanner geometry to avoid artifacts and blurring in reconstructed images. Furthermore, in some volumetric imaging systems, it is necessary to physically adjust the locations and/or orientations of the various components to properly align the imaging system.
To obtain good qualitative, or even quantitative reconstruction images, parameters of each viewpoint must be accurately known. A viewpoint pertains to the location and orientation of the various system components with respect to the object. Estimating the parameters directly, i.e. for example by making direct measurements on the acquisition system, is often very difficult and the results can be imprecise.
The term “geometrical calibration of an imaging system” denotes the operation that leads to precise indirect knowledge of the geometrical parameters (well known geometrical parameters include but are not limited to, tube and/or detector position and/or orientation, relative to the imaged object, or relative to a reference coordinate system) that play a role in the production of an image. The underlying principle is based on the use of a geometrical phantom that is (at least partially) known in the 3D space and whose projection image is acquired. The geometrical parameters are then identified based on information extracted from the image.
For single slice CT scanner, it is known that all relevant parameters for alignment can be determined from a single scan of one or two point-like objects or pins. “Pin scans” can be used to extract the magnification of a CT system as well as the center of rotation in a straightforward manner. This technique is specific to single slice CT scanners, since it assumes certain characteristics of the scanning trajectory. It is, however, not applicable to volumetric scanners, for which a number of additional parameters are required.
For high quality 3D reconstruction from a set of projection images, e.g., in X-ray, very accurate information about the acquisition geometry for each projection image is required. In CT, for example, this information is typically available, since the geometric specifications of the gantry are well known, and the mechanics of the gantry and the synchronization with the image acquisition are tightly controlled. However, in other scenarios, e.g., the acquisition of projection data with systems that were originally designed for pure 2D imaging, the system mechanics may be less well defined (e.g., due to mechanical deformability of the gantry), and the synchronization of the image acquisition with the gantry position may also be less accurate. If, however, the geometry of a 3D spin acquisition is repeatable (although not accurately known beforehand, without a calibration), or if the calibration data can be acquired concurrently with the image information of the imaged object, then the conditions for a calibration according to the current invention, are satisfied. This general scenario may apply to mobile C-arm systems (as typically employed for surgical imaging), to fixed-room C-arm systems (e.g., systems for cardiovascular imaging), and also to tomosynthesis-type imaging systems. In these situations, a geometric system calibration may be required for generating images of improved quality.
Current techniques for calibrating/aligning volumetric scanners include the use of phantoms of special construction. These phantoms use a series of small physical balls (e.g., spherical “BBs”) or markers in a well-defined, highly accurate spatial configuration, thus allowing for a full geometry calibration for a single view (e.g., calibration using a helix phantom where BBs are located on a helix at a surface of a cylinder, which is placed such that the axis of the cylinder approximately coincides with the axis of rotation of the gantry), as well as other, similarly configured phantoms, maybe containing a smaller number of markers, that allow for a constrained calibration in conjunction with additional system information (e.g., ring containing BBs, where object/anatomy to be imaged is placed within the ring such that partial geometry information is acquired concurrently with the projection images to be used for the 3D reconstruction). The projection image of the phantom may then be used to extract the exact system/imaging geometry at each view position, thus providing the required geometrical information for image reconstruction or system alignment (if the phantom uses a sufficient number of BBs, and the phantom is otherwise matched to the system geometry). However, such phantoms and the associated computational approaches work reliably only over a limited range of imaging geometries (tube and detector position and orientation). In particular, the diameter of the cylinder and pitch of the helical matrix of the phantom limit the utility of such phantoms to a narrow range of magnifications and cone angles.
It is therefore desirable to provide phantom design and calibration methods for volumetric imaging systems to determine the acquisition parameters with greater accuracy so as to generate better images.